Trisomy 17 mosaicism is a chromosomal abnormality in which there are three copies of chromosome 17 in some cells of the body, rather than the usual two copies. Trisomy 17 mosaicism is one of the rarest trisomies in humans. It is often incorrectly called trisomy 17 (also referred to as full trisomy 17), which is when three copies of chromosome 17 are present in all cells of the body. Full trisomy 17 has never been reported in a living individual in the medical literature. Few cases of trisomy 17 mosaicism have been described, most having been detected during pregnancy through a test called amniocentesis. Only a few individuals have had a confirmed diagnosis of trisomy 17 mosaicism after birth. Because the proportion and location of cells with trisomy 17 differs from case to case, the presence and severity of signs and symptoms may vary significantly from person to person.

Trisomy 17 mosaicism occurs due to a random event during the formation of the reproductive cells (egg and sperm) or after fertilization has taken place. An error in cell division (called nondisjunction) may cause some eggs or sperm to have an abnormal number of chromosomes. If an egg or sperm with an extra chromosome 17 contributes to the genetic makeup of an embryo, the embryo will have an extra copy of chromosome 17 in each cell. As the embryo grows and divides, an attempt may be made to correct the mistake by eliminating one extra chromosome 17. In individuals with trisomy 17 mosaicism, this attempt may be partly successful, leaving some cells with an extra chromosome 17 and some cells with the extra chromosome deleted (the usual chromosome number). This correction process is called trisomy rescue. In other cases, the egg and sperm may have a normal number of chromosomes, but an error of cell division (nondisjunction) occurs when the fertilized egg is growing and dividing. If an error occurs during one of the divisions, it can cause some cells to have an abnormal number of chromosomes. In people affected by trisomy 17 mosaicism, some of the body's cells have the usual two copies of chromosome 17, and other cells have three copies of this chromosome (trisomy). The percentage of cells with trisomy 17 and which parts of the body are affected vary from person to person. This leads to variability in the range and severity of symptoms. MedlinePlus, a resource through the National Library of Medicine, offers general information on mosaicism.

While trisomy 17 mosaicism is rare, there are a few cases that have been reported in the medical literature as well as a few reviews. Most cases of trisomy 17 mosaicism reported in the medical literature that were detected during pregnancy via chorionic villus sampling (CVS) or amniocentesis have not been confirmed in the baby after birth. In those cases, the babies appeared to be in good health at birth and it is suspected that in those cases, trisomy 17 was confined to extra-embryonic (placental) cells and tissues. Some cases of trisomy 17 mosaicism detected during pregnancy have been confirmed in the baby after birth. The symptoms reported include: developmental delays, body asymmetry, slow growth, and cerebellar hypoplasia. Again, signs and symptoms may vary in these individuals depending on which cells and how many cells contain an extra chromosome 17.